Abstract

We report the first finding of a mica megacryst included in an alkaline basic dyke belonging to the Tertiary Veneto Volcanic Province (northern Italy). The megacryst is surrounded by a corona with a mineral assemblage consisting of fine-grained mica flakes + Ti-magnetite. The megacryst is a biotite with a mg# of 0.75, containing 1.35 wt. % TiO2, 1041 ppm Ba and 3.8 wt. % H2O. The mica flakes have a mg# of 0.79 and contain 4.33 wt. % TiO2,21 968 ppm Ba and 3.0 wt. % H2O. Both mica types show high concentrations of LILE and Nb (461–135 ppm). REE contents are typically below the analytical detection limit. Fractional crystallization modelling shows that the mica megacryst crystallized at depths in the mantle consistent with the spinel-facies stability field. The unusually high NbN/SrN and SrN/ZrN < 1 observed in the megacryst can be explained by a hydrous K-Fe-rich parent liquid that has already undergone fractional crystallization of Sr-bearing phases such as apatite and plagioclase.

The calculated structural formulae based on 24(O+F+Cl) indicate that full occupancy of the tetrahedral site can only be attained by allowing the entry of a small fraction of Fe3+ or Ti4+. In addition, Ti-oxy (likely coupled with Ba-oxy) substitution occurs to provide charge balance with the excess negative charge at the O4 position. The occurrence of such exchange mechanisms can enhance the thermal stability of the mica flakes that most likely crystallized at shallower depths under high temperature and high f(O2) conditions.